Update: The adCenter blog posted that they made an update to the algorithm that ranks the ads. They now take into account relevancy and quality factors such as "content of the ad and landing page, keywords that an advertiser selects in relation to the advertiser’s landing page content, substantive content on the landing page, and duplicative nature of content in overall search results."

A WebmasterWorld thread has very early reports of an unusual drop in traffic from live.com and MSN adCenter. Here are the two reports both stating a drop of about 75% of their traffic from Microsoft's PPC engine, adCenter.

Just noticed a 75% drop in my MSN traffic yesterday. At first I thought It might have been a resuly of changing ads on my account but now I believe its more of a system problem.

Yes big drop around 7 PST. More than 75%. Both UK and US

This is still very early, so it may be nothing or it may be the signs of something big happening with adCenter.